The present invention deals with a process control system. More specifically, the present invention deals with a terminal block in a process control transmitter.
Process control transmitters typically include a sensor for sensing a process variable, such as pressure, flow, or other process variables. The sensor provides a sensor output signal which is indicative of the process variable. Many such transmitters also typically include processing circuitry which receives the sensor output signal and corrects and linearizes it based on certain errors and non-linearities which may occur in the sensor output signal. The corrected signal is then provided to processing and transmitter electronics in the process control transmitter which transmits a signal indicative of the sensed process variable to a remote location, such as to a control room or a field mounted controller.
Such transmitters are often field mounted devices and, as such, require a variety of terminal connections to be made during installation. Such terminal connections often include connection to a communication loop (such as a 4-20 mA loop), and connection to a power supply for a total 4-wire connection.
Also, since such transmitters are often field mounted devices, they are exposed to large transient signals such as those induced by lightning strikes. In addition, they are often placed in hazardous environments and are thus required to meet certain intrinsic safety standards, explosion proof criteria and standards governing the size and spacing of terminal connectors.
Therefore, in process control transmitters, providing a terminal block which enables efficient installation of terminal connections, and which still meets intrinsic safety, explosion proof, and size and spacing standards, can pose significant problems.
In addition, in hazardous environments it is highly desirable to take measures which tend to inhibit the discharge of electricity which could cause a spark. Thus, it is often desirable to have some type of covering over the terminal connections which are used to connect the transmitter to the power supply. Such covers are provided in order to prevent the accidental contact of both terminal connections by a tool or other implement used by an installation or service person, which could induce a spark. Prior terminal blocks provided a variety of covers.
One prior cover is configured as a hinge or swinging type cover. The cover includes a strip of non-conductive material which extends over the power supply terminal connectors and which is pivotable about one end thereof. Thus, the cover can be swung or pivoted from a covering position in which it covers both power supply terminal connectors to an accessing position in which both power supply terminal connectors are exposed.
Another prior cover is configured to slide relative to the power supply terminal connectors. The cover is configured to slide from a covering position in which both supply terminal connectors were covered to an accessing position in which both power supply terminal connectors were exposed. However, both prior power supply terminal connector covers have significant drawbacks. First, both prior covers either cover both terminal connectors or expose both at the same time. Therefore, there is a possibility that the two terminal connectors can be shorted together to cause a spark. Further, both prior covers require significant additional space within the transmitter enclosure to move from the respective covering positions to the accessing positions. Such space is not often available but is usually quite limited in typical process control transmitter housings.
Also, typical transmitters may have fused circuits. Since the transmitters are often field mounted transmitters, they are typically located in areas that are very difficult to access by service and installation personnel. The areas containing the transmitters may not be well lit, and can also contain other structures which hinder access to the transmitters. Thus fuses in prior transmitters can be very difficult to change. Also, manipulating the connections to the transmitter can also be very difficult because of the impediments to access.